The present invention relates to a method of treating particles, and, more particularly, to a method of treating spherical or spheroidal semiconductor particles, after they have been subjected to melting or heating, so as to separate adjacent particles which adhere or stick together because of the prior melting or heating. In specific embodiments, the particles are silicon spheres or spheroids which are used to fabricate solar cells.
One type of solar cell or photovoltaic device includes a plurality of spherical or spheroidal semiconductor particles which extend away from both sides of a first flexible metal foil sheet and are affixed to the walls of apertures formed in the foil sheet. The details of the construction (i.e., the mechanical and electrical form, fit and function) and fabrication methodology of this type of solar cell may be found in the following commonly assigned U.S. Pat. Nos.: 5,192,400; 5,091,319; 5,086,003; 5,028,546; 4,994,878; 4,992,138; 4,957,601; 4,917,752; 4,872,607; 4,806,495; and 4,691,076.
The aforenoted fabrication methodology utilizes generally unisized spheres of a semiconductor material, such as silicon, each having a p-n junction, which spheres are produced by one of a number of techniques. See, for example the production methods disclosed in commonly assigned U.S. Pat. Nos. 4,430,150; 4,637,855; 5,012,619; and 5,069,740. The spheres are typically constituted of an outer silicon portion or shell of one conductivity type surrounding an inner silicon portion of the other conductivity type, both portions having a selected purity and other relevant characteristics. The spheres are capable of producing electricity when radiation, such as solar radiation, is incident thereon. The produced electricity may flow between conductors, one of which is electrically continuous with the outer portion of each sphere, and the other of which is electrically continuous with the inner portion of each sphere. In the aforenoted patents, these conductors are preferably flexible foils of a metal such as aluminum, to the first of which the outer portions of the spheres are affixed, as noted above.
A contemplated method for manufacturing solar cells begins with forming in the first aluminum or other metal foil sheet a pattern of apertures, the diameters of which are slightly less than the diameters of an available quantity of same-sized silicon or other semiconductor spheres. One method for forming the apertures includes first embossing and then etching the foil sheet. After formation of the aperture pattern, the spheres are loaded onto the foil so that each aperture is overlain by a sphere. Because of the relative sizes of the diameters of the same-sized spheres and the apertures, the aperture-located spheres merely nest in their respective apertures on one side of the foil sheet without substantially protruding through the other side of the foil sheet.
The spheres are then mechanically and electrically affixed and connected to the first foil. Such affixation and connection is achieved by applying suitable compressive forces to the foil-sphere system, as set forth in the above-noted patents. Typically the application of the compressive forces is achieved by the use of a press which acts on the spheres and the foil through selected compliant and rigid elements which are positioned between working surfaces of the press and the foil-sphere system. These elements prevent damage to the spheres and to the foil, while ensuring that the applied forces effectively move the spheres partially through their respective apertures.
Partial movement of the spheres through their respective apertures effects mechanical affixation of the spheres to the walls of their apertures and renders electrically continuous with the first foil the outer surfaces of the spheres. These ends are achieved, in part, through the relationship of the larger diameters of the spheres to the smaller diameters of the apertures. This relationship directly results in the mechanical and electrical affixation and aids in effecting the electrical continuity of the spheres with the first foil. When the spheres are moved partially through the apertures, the edges of the aperture walls and the surface of the spheres mechanically interact and mutually abrade each other to remove any natural oxide on the spheres or the aperture walls. Thus, a metal-sphere (i.e., an aluminum-silicon) bond is formed. The foregoing may be enhanced by the application of heat during the compression.
The outer portion of one conductivity type of the located and affixed spheres is removed, as by etching, from the spheres. This removal occurs only on one side of the first foil sheet to expose the inner sphere portions of the opposite conductivity type. An electrically insulative layer is applied or deposited on the exposed inner sphere portions and the one foil sheet side. Small regions of the layer which overlie the exposed inner sphere portions are removed, as by abrading or etching, to create openings or vias through which access to the inner sphere portions may be obtained. A second flexible metal (e.g., aluminum) foil is mechanically and electrically connected to the inner portions of the spheres through the openings or vias by thermo-compression bonding or a functionally equivalent technique.
The solar cell is now nearly complete. Radiant energy directed toward the free surface of the first foil falls on the spheres which produce electricity. A utilization device is connected between the foils. The electricity flows from one portion, inner or outer, of the spheres through one of the foils, through the utilization device and ultimately through the other foil into the other portion, outer or inner, of the spheres. The insulative layer electrically insulates the foils from each other. The flexible cell may be conformed to a desired surface or shaped in selected fashion. A protective cover may be placed over or applied to the spheres. The cover may include or comprise lenses Which direct an increased amount of incident radiant energy onto the spheres to increase the efficiency of the cell.
Typical silicon sphere production techniques may result in batches of intermingled silicon spheres or spheroids which are adherent or stuck together. Specifically, the production of silicon spheres usable in the solar cells of the foregoing patents is exemplified in commonly assigned U.S. Pat. Nos. 4,430,150; 4,637,855; and 5,069,740. Silicon sphere production involves the heating and melting, often repetitive, of particulate metallurgical grade silicon starting material or feed stock. Melting of the silicon is preceded by the formation about each silicon particle of a skin of a material such as silicon dioxide. Melting of the metallurgical grade silicon particles results in impurities therein traveling to the previously formed skin and in each particle of the melted silicon being configured by surface tension into a sphere or spheroid. The skin is sufficiently plastic to permit the assumption of this configuration without rupturing. The skin-encased silicon spheres or spheroids are then controllably cooled until they resolidify. Subsequently, the skins are removed from the resolidified silicon spheres, which have a higher purity than metallurgical grade silicon. Repetitive effectuation of the process results in semiconductor grade silicon spheres.
Due to their proximity while they are molten, adjacent spheres or spheroids may, at times, adhere together after resolidification. Since the spheres are intended to be used individually in fabricating solar cells, or are intended to be individually further processed before such use, an object of the present invention is the provision of a method of and apparatus for treating the spheres or spheroids by breaking apart adherent ones thereof.